Validation error towards Crossref only with xmltools

[molnart-gh]

From several updates ago the xml tools (3.1.1.12 in Notepad++ 32bit) has not not validate my xml by indicate error in doi_batch tag:

<?xml version="1.0" encoding="utf-8"?>
<doi_batch xmlns="http://www.crossref.org/schema/5.3.1"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:jats="http://www.ncbi.nlm.nih.gov/JATS1"
xmlns:fr="http://www.crossref.org/fundref.xsd"
xmlns:mml="http://www.w3.org/1998/Math/MathML"
xsi:schemaLocation="http://www.crossref.org/schema/5.3.1 https://www.crossref.org/schemas/crossref5.3.1.xsd"
version="5.3.1">

The error message something like is: Repeated node : name = '{http://www.w3.org/1998/Math/MathML} ContExp'. So the validation process is hang up after this line.

The xmls can be submitted to Crossref, so they are well-formed and valid anyway, but if I want to check the validation correctness I should use another software. I have contacted the Crossref support, there is no error on their side. Pls. check it. Thank you. If you need I can send a full and valid xml.